1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus which reduces cross talk caused by adjacent tracks occurring when a high capacity recording medium having a narrow track pitch is recorded or reproduced.
2. Description of the Related Art
In order to increase the capacity of a recording medium, development is centered on using a light source of a shorter wavelength and an objective lens having a larger numerical aperture when recording/reproducing the recording medium. That is, as an optical recording medium, a compact disk (CD) which uses an infrared light source of a wavelength of 780 nm and an objective lens having a numerical aperture of 0.45 is replaced by a digital versatile disk (DVD) which uses a light source of a wavelength of 650 nm and an objective lens having a numerical aperture of 0.6. In addition, to change specifications from the CD to the DVD and enhance the density of recording, development is centered to reduce the track pitch of a recording medium.
However, in the case of a recording medium having a narrow track pitch, the cross talk between adjacent tracks may deteriorate the reproduction of a recorded signal.
When a recording medium is recorded/reproduced by an optical pick-up apparatus, the degree of deterioration of the signal by adjacent tracks is defined by cross talk between the tracks, and an allowable value is set according to a recording medium. For example, in order to acquire a high quality reproduced signal in the case of a DVD-ROM, the DVD is required to have a cross talk value less than xe2x88x9230 dB.
A DVD-ROM uses an objective lens of a numerical aperture of 0.6 and a light source of a wavelength (xcex) of 650 nm, is manufactured to have a track pitch of 0.74 xcexcm, and has a capacity of 4.7 GB. Since the size of a beam spot determining a recording capacity is proportional to xcex/NA, in the above case, xcex/NA is 1.08. Therefore, the ratio of the track pitch (Tp) to the beam spot is Tpxc3x97NA/xcex=0.68, and the cross talk between adjacent tracks has a value lower than xe2x88x9230 dB.
Since an HD-DVD which is expected to attract public attention as a high density recording medium in the near future requires a capacity greater than 15 GB, the ratio of the track pitch to the beam spot must be lower than 0.6, and in this case the cross talk caused by adjacent tracks increases. For example, when a HD-DVD uses a light source of a wavelength of 410 nm and an objective lens of a numerical aperture of 0.6, and the track pitch is set to 0.368 xcexcm so as to have a capacity of 15 GB, cross talk caused by adjacent tracks has a very high value, higher than xe2x88x9220 dB. Therefore, in order to prevent deterioration of a reproduced signal, an optical pickup apparatus which is adapted to reduce cross talk caused by adjacent tracks is required.
FIG. 1 shows a conventional optical pickup apparatus capable of reducing cross talk between adjacent tracks.
Referring to FIG. 1, a light emitted from a light source 1 is diffracted by a grating 2, and is separated into three light beams of a zeroth order diffracted light beam and xc2x1 first order diffracted light beams. After the beams are reflected on a beam splitter 3, the reflected beams are converged by an objective lens 5 and form three light beam spots S1, S2 and S3 at positions different from each other on an optical disk 10 as shown in FIG. 2. That is, the zeroth order diffracted light beam forms the light beam spot S1 on a main track T1 from which an information signal is reproduced, and the xc2x1 first order diffracted light beams form the light beam spots S2 and S3 on first and second adjacent tracks T2 and T3 adjacent to the main track T1, respectively.
At this time, as shown in FIG. 2, the light beam spots formed on the optical disk 10 are formed on the main track T1 and the adjacent tracks T2 and T3 to be slanted with respect to each other. That is, the light beam spot S2 formed on the first adjacent track T2 precedes the light beam spot S1 formed on the main track T1, and the light beam spot S3 formed on the second adjacent track T3 goes behind the light beam spot S1. Here, since side portions of the individual light beam spot lie over adjacent tracks due to the narrow track pitch, the zeroth order diffracted light beam and the xc2x1 first order diffracted light beams form the light beam spots not only on the corresponding tracks but also on the adjacent tracks.
The light beams which were reflected from the optical disk 10 and passed through the objective lens 5 pass through the beam splitter 3 and are detected by a photodetector 8 which is provided with first, second and third light receiving portions A, B and C performing photoelectrical conversion independently of each other. That is, the three light beams reflected from the respective tracks of the optical disk 10 are received by the respective light receiving portions, A, B and C of the photodetector 8 as shown in FIG. 3.
In FIG. 1, reference number 4 denotes a reflective mirror, and reference number 7 denotes a sensing lens which condenses incident light beams on the photodetector 8.
In the optical pickup apparatus structured as described above, an information signal to be detected is a signal which is reflected from the main track T1 and received by the second light receiving portion B. However, since portions of the light beam spot S1 by the zeroth order diffracted light beam are formed over the first and second adjacent tracks T2 and T3, the detected signal of the second light receiving portion B includes not only the signal of the main track T1 but also the signals of adjacent tracks, which are intermixed therewith. Therefore, the information signal of the main track T1 is detected by performing a differential operation on the detected signal of the second light receiving portion B and the signals of adjacent tracks detected by the first and third light receiving portions A and C. When the light receiving portion and the signal detected by the light receiving portion are expressed by the same symbol, the RF signal of the main track T1 is expressed by the following Equation (1)
xe2x80x83RF signal=Bxe2x88x92Kxc3x97[A+C]xe2x80x83xe2x80x83(1)
where K is a constant which is determined so that vibrations in the time axis of the information signal of the main track T1 can be minimized, i.e., a constant which is determined so that cross talk by the adjacent tracks T2 and T3 can be minimized.
On the other hand, since the three beams diverged by the grating 2 are converged on respective tracks to form the light beam spots which are slanted with respect to each other as shown in FIG. 2, the signals of the adjacent tracks which are detected by the first and third light receiving portions A and C are delayed for a constant time with respect to the signals of the adjacent tracks which are included in the signal detected by the second light receiving portion B. Therefore, it is not possible to perform a real-time operation so as to detect the information signal of the main track, and there is a problem in that signals which are delayed for the constant time with respect to the detected signal B are used as the signals A and C of the adjacent tracks.
In addition, in the above method of reducing cross talk by adjacent tracks, since three light beam spots must be formed on respective tracks of the optical disk 10, the optical efficiency of the light beam spot S1 for recording/reproducing the information signal of the main track T1 is lowered, and it is difficult to use the light beam spot S1 for recording.
To solve the above problems, it is an objective of the present invention to provide an optical pickup apparatus which detects, in real-time, an information signal of a main track in which cross talk caused by adjacent tracks is reduced, and which reduces cross talk by adjacent tracks without lowering the optical efficiency of a light beam spot.
Accordingly, to achieve the above objective, there is provided an optical pickup apparatus which reduces cross talk by adjacent tracks including: a light source for emitting a light beam; a light path changing means disposed on the optical path between the light source and a recording medium for changing the path of an incident light beam; an objective lens disposed on the optical path between the light path changing means and the recording medium for converging an incident light beam onto the recording medium; a photodetecting means dividing the incident light beam which is reflected from the recording medium and then passes through the light path changing means into a center portion and side portions with respect to the radial direction of the recording medium and receiving the incident light in the divided form; and an operation unit for operating the detected signals of the center portion and the side portions of the incident light beam from the photodetecting means and outputting an information signal of a main track reduced in cross talk by adjacent tracks.
According to one aspect of the present invention, the photodetecting means which is a photodetector comprises a main light receiving portion which receives the center portion of the incident light beam, and at least one auxiliary light receiving portion which is disposed at a side of the main light receiving portion with respect to the radial direction of the recording medium receive the side portions of the incident light beam apart from the main light receiving portion.
Preferably the main light receiving portion has a predetermined size so as to receive the center portion within a range of about 10 to 90% of the incident light beam.
According to another aspect of the present invention, the photodetecting means comprises: an optical member for directly passing the center portion of the incident light beam and for diffracting the side portions of the incident light beam at a predetermined angle, thereby separating the incident light beam into the center and side portions in a radial direction; and a plurality of photodetectors for photoelectrically converting the separated light beams into electrical signals independently of each other.
Preferably the operation unit includes: a multiplier which multiplies the detected signals of the side portions of the incident light beam by an adjusting operation constant K so that cross talk by adjacent tracks can be minimized; and an adder which adds the signal output from the multiplier and the detected signal of the main light receiving portion of the incident light beam, and outputs an information signal in which cross talk is reduced.